The present invention relates generally to light detectors that are relatively insensitive to the angle of incident light over a predetermined spherical sector and, more particularly, to light detectors that uniformly detect light from any direction in a hemisphere and provide a measurement of the light's intensity.
A planar photodiode is an example of a light detector that is sensitive to the angle of incident light. The photodiode's planar or flat surface exhibits a maximum cross-sectional area to light that is incident at an angle normal to the flat surface. However, as the angle of incident light increases from the normal, the cross-sectional area of the flat surface decreases as a function of the cosine of the angle. Accordingly, the planar photodiode's response is related to the angle of the incident light by the angle's cosine.
Further, photodiodes, like most photodetectors, suffer from an additional effect that depends on the angle of the incident light called Fresnel reflection. Fresnel reflection generally occurs whenever light travels through a surface between two materials having different indices of refraction, for example, air and glass or silicon. As the incident light angle increases from the normal, the Fresnel reflection also increases, which decreases the amount of light actually entering the detector.
Currently known hemispherical light detectors generally employ either a translucent diffuser or a multi-element detector system. The translucent diffuser is a sheet of translucent material placed over the photodiode's surface. Incident light passes through the translucent sheet and is diffused over a large angle, a portion of which is intersected by the photodiode. The translucent diffuser can be made of many materials including ground glass, acrylics, or Teflon. However, the translucent diffuser fails to eliminate the cosine effect discussed above and, because the photodiode intercepts only a portion of the diffused light, it is generally inefficient. Multi-element detectors employ several photodiode elements that are each configured to cover a predetermined spherical sector. Coverage over a larger spherical sector is obtained by combining the signals from the several detectors. While multi-element detector systems are capable of providing hemispherical coverage, their reliance upon multiple elements dramatically increases the cost and complexity of the system. The effectiveness of the multi-element detector systems is also lessened due to non-uniformities which occur from transitions from one element to the next.
Accordingly, there is a need for a light detector which is relatively insensitive to the angle incidence of a light source in a relatively simple and cost effective manner. The present invention satisfies this need.